# Reference attribute

When sending SMS messages, you can include an optional `reference` object to track messages and associate them with specific services or actions. All [`send`](/products/sms/enterprise-documentation/developer-documentation/api-references/send) methods accept this object.

| Parameter | Type | Required | Description |
|  --- | --- | --- | --- |
| `reference.service` | string | Required if any other `reference` property is specified | Identifies the product or service sending the SMS. Must match one of the predefined allowed values — any other value returns an error. |
| `reference.action` | string | No | Describes the specific action for the selected service (e.g. `2FA`, `verification`). |
| `reference.key` | string | No | Custom tracking key for your internal processes. |


#### Example


```json
{
 "destination": "+1234567890",
 "text": "Your verification code is: 123456",
 "reference": {
   "service": "AUTHENTICATION",
   "action": "2FA",
   "key": "user-12345"
 }
}
```